The air induction system (AIS) of an internal combustion engine of a motor vehicle, see generally 10 in FIG. 1, is a complex and finely-tuned system, balancing requirements and constraints that are often in tension with one another.
Conventionally, an air cleaner with a mass air flow sensor package 10a is connected to the engine intake manifold (not shown) by an air cleaner outlet duct assembly 12 which includes a convolute duct 14. Convolute air ducts of various lengths and sizes are used to transfer clean, filtered and metered air from an air filter to the engine air intake manifold. Oftentimes, these convolute ducts are of a complicated serpentine configuration due to limited space within the motor vehicle engine compartment.
The outlet duct assembly must fulfill the following main functions: provide a smooth airflow with minimum flow restriction (pressure loss); absorb engine motions under engine torque roll in all operating conditions; isolate engine vibrations from transmitting to the body structure; resist collapse under vacuum; maintain sealing at all system connections; and provide duct flexibility to ease efforts of the air filter service.
It is a challenge to keep a proper balance as between all of these needs because many of the requirements are conflicting. For example, to accommodate engine movement, the outlet duct assembly 12 must have flexibility, the flexibility being provided by convolutes 16 in the sidewall 18 of an air duct to thereby provide a convolute duct 14 having a convolute portion 14a and non-convolute portions 14b on either side thereof.
Convolute ducts are generally formed using an injection or blow molding process. The convolute duct is normally made from soft materials, such as a thermoplastic elastomer (TPE), or rubber. The flexibility depends on the parameters of convolute duct like material hardness, wall thickness, radius, and length. The convolutes 16 of the convolute duct 14 are characterized by a plurality of raised circumferential ridges 20 (see FIG. 1B) which allow compression, extension, deflection and distortion of the convolute duct sidewall 18. Thereby, a convolute duct performs as a decoupling agency for assembly, engine movements, shock absorption and noise, vibration and harshness (NVH) control.
Though convolute ducts 14 are effective in absorbing engine motions and isolating vibrations, a problem with convolute ducts is that the internal circumferential ridges 20 of the convolutes 16 have large surface roughness, and each provide a cavity 22 adversely affecting local air flow. The surface of the ridges 20 and the cavities 22 create flow resistance and turbulent counterflow air CF which opposes the air flow F, thus increasing pressure loss through the convolute duct 14. This frictional resistance is known in the art as the “friction pressure loss”. In particular, highly convolute ducts with compound angles can further restrict air flow, due to large flow turbulence.
The amount of friction pressure loss across a convolute duct is based in part upon the fluid characteristics, such as the fluid's density, the fluid's viscosity and the fluid's flowing rate. However, most importantly, the surface roughness of the convolute duct sidewall has the biggest effect on the friction pressure loss.
Since the convolutes have internal circumferential ridges, with large surface roughness, when airflow passes through the convolute duct, airflow is strongly affected by this surface roughness. Surface roughness is a defining feature of many of the high Reynolds-number fluid flows known in fluid dynamics engineering. In fact, the higher the Reynolds number (Re), the more likely the effects of roughness will be significant. The friction factor of a rough wall can be calculated by the formula:
                              1                      f                          1              /              2                                      ≈                              -            1.8                    ⁢                                          ⁢                      log            [                                          6.9                Re                            +                                                (                                                            ɛ                      d                                        3.7                                    )                                1.11                                      ]                                              (        1        )            where f is the friction factor of the rough wall, Re represents the Reynolds number, ε represents the wall roughness height (e.g., convolute height in this case), and d represents the duct diameter. Equation 1 shows that the higher convolutes height ε, the larger is the wall roughness friction.
For the flow in a circular duct, the head pressure loss can be expressed as:
                              h          f                =                  f          ⁢                      L            d                    ⁢                                    V              2                                      2              ⁢              g                                                          (        2        )            where L is the duct length and V is the average velocity of airflow in the duct.
The pressure loss for a horizontal duct is:Δp=ρghf  (3)
Combining Equations 2 and 3, the pressure loss due to the wall-roughness friction can be derived:
                              Δ          ⁢                                          ⁢          p                =                  f          ⁢                                    ρ              ⁢                                                          ⁢                              LV                2                                                    2              ⁢              d                                                          (        4        )            
From Equation 4, it is indicated that the pressure loss is directly proportional to the wall-roughness friction factor f. Therefore, in order to reduce the pressure drop, it is critical to minimize the wall-roughness friction factor. From Equation 1, reducing convolute height would decrease the wall friction, but this would conflict with the duct flexibility requirement. In order to absorb engine roll motions and decouple engine vibrations from vehicle body structure, the convolute duct must have sufficient flexibility with a minimal 12 mm convolute height. This size of convolute height will result in large friction factor according to Equation 1. For example, for a 150 mm long convolute duct with an 80 mm diameter and a 12 mm convolute height, the friction factor is 0.089 at 200 g/s flow rate. The wall roughness friction can result in 0.1 KPa friction pressure loss, based on the calculations per Equations 1, 2, 3 and 4.
It is well known that the more air flow that can be delivered to the engine, the engine will be able to generate more power. The pressure loss across the convolutes of a convolute duct means less air is pushed into engine. Consequently, engine power output is reduced as a result of pressure loss caused by a convolute duct.
An alternative to reducing pressure loss in a convolute duct is to decrease the convolute quantity and shorten the convolute duct. Although less convolutes will be able to reduce the air friction resistance, it still has inherent problems. First, less convolutes will lessen the ability of the duct to absorb engine vibrations. Engine vibrations can be easily transmitted to the body structure. Consequently, the excessive vibrations from the engine could cause discomfort to drivers and passengers. Secondly, each convolute has to take more engine positional displacements due to less convolutes. As a result, the durability of a short convoluted duct is always a significant concern. Finally, a short convolute duct needs more assembly effort in the vehicle assembly plant. Commonly, this type of duct can cause human ergonomic issues, thus increasing the assembly cost and time.
Another approach is to design the convolute duct with a large inner diameter. Although lowering the pressure loss within the convolute duct, the large diameter convolute duct has several limitations. As a general rule, a convolute duct must keep a 25 mm clearance to other components within an engine compartment, known in the art as “dynamic clearance” which ensures there is no hard contact between the convolute duct and surrounding components under engine torque roll conditions. Therefore, the size and routing of the convolute duct are restrained in the tight engine compartment of today's motor vehicles. Furthermore, the engine throttle body and mass air flow sensor have relatively small diameters due to their standard design dimensions. If the convolute duct diameter is too large relative these components, the air flow can experience sudden expansion at the exit of the mass air flow sensor and a sudden contraction at the entry of the engine throttle body. This sudden expansion and contraction of the air flow can cause significant pressure loss.
Accordingly, what remains needed in the art is an air induction system for an internal combustion engine of a motor vehicle in which the outlet air assembly includes a convolute duct which avoids its inherent frictional pressure losses.